1. Field of the Invention
The present invention relates to a sheet material used for magnetic wave shield, and more particularly to a composite sheet excellent in electromagnetic wave shielding effect.
2. Description of the Related Background Art
As electronic devices such as computers, office automation instruments, and the like become popular nowadays, there are many chances for electromagnetic waves generated by such devices to cause an error operation in such an electronic device. There have been various methods studied to protect the electronic devices from the electromagnetic interference, for example a method to form walls and floors in housing or room with an electromagnetic wave shielding material, a method to stick a sheet for electromagnetic wave shield to walls and floors, or a method to cover wiring inside a room or a car with a metal foil.
Many materials are proposed as such a sheet material for electromagnetic wave shield, for example, metal foils, carbon fiber sheets or carbon fiber fabrics, and synthetic resin films in which powders or fibers of a conductive material such as metal oxides, metals, or conductive carbons, and so on are intermixed. It may be also known that an adhesive layer is continuously or intermittently given on one side of such a sheet material for electromagnetic wave shield to form a wall covering or to form a decorative laminate using the wall covering.
Among such sheet materials for electromagnetic wave shield, a sheet in which a thermoplastic synthetic resin layer, acting as the adhesive layer, such as a polyester film is provided on a back surface of a metal foil such as an aluminum foil is relatively light in weight and easy in processing, and therefore becomes often used in constructing a shield room or in shielding an electronic device. However, since the synthetic resin layer of the sheet is non-conductive, secondary treatments are necessary to connect between two sheets in laying with a conductive coating or a conductive adhesive and further to earth the sheets in order to obtain a satisfactory electromagnetic wave shielding property. Further, the sheet also has a problem of ineffectiveness to magnetic field shield, though it is effective to static charge prevention and electric field shield.